1. Field of the Invention
This invention relates to helicoidally finned tubes and more particularly to heat exchanger tubes of such type.
2. Description of the Prior Art
As is known, heat transfer between fluids of different heat transfer coefficients is obtained, among other ways, by means of helicoidally finned tubes which consist of an inner tubular member and an outer helical member. The turns of the helical member form the fins of the tubes. The fluid of greater heat transfer coefficient such as liquids or condensing vapours flows in the tubular member. The fluid of smaller heat transfer coefficient such as gases or air flows between the turns--the fins--of the helical member at right angles to the longitudinal or principal axis of the tubular member and, thus, to the finned tube itself.
Helicoidally finned tubes having solid helical surfaces the generatrix of the turns of which is at right angles to the axis of the tubular member are already known. Such geometry permits adopting simple manufacturing methods which consist either in winding and fixing a band of rectangular or L-shaped cross sectional configuration onto the tubular member or in die-rolling helical ribs from the body thereof. In the latter case the turns of the helical member have outwardly diminishing cross sectional areas which means outwardly increasing gaps between the fins. In either case heat transfer is uneven along the radial extent of the fins which is undesirable for thermodynamic reasons because it results in relatively low mean temperatures of the exiting external fluids as will immediately be explained:
If, for instance, the tubular member has a fluid flowing in it which is warmer than air, the temperature of the fins decreases with increasing distances from the tubular member. At the same time the flow rate of air increases in the same direction because in the gaps between the fins less air will flow in the proximity of the tubular member than farther out. This is due to inwardly increasing flow resistances met by the external fluid. Namely, the flow path of air is longer in the central regions of the fins than at the periphery thereof. In addition, air flowing at the base of the fins contacts the outer surface of the tubular member, in contrast to the air flowing at the periphery which sweeps the side surfaces of the fins only. Such difference is even more pronounced with tubes having die-rolled fins where besides a radial and outward decrease of flow path lengths also the gaps between adjacent fins widen towards the periphery thereby augmenting the cross sectional flow area of air and diminishing the flow resistance thereagainst.
Thus, air flow in the gaps between adjacent fins is uneven which is responsible for the already mentioned low values of the mean temperature of the exiting air.